- Email: [email protected]
Investigations on hydrogenation behaviour of CNT admixed Mg2Ni
international journal of hydrogen energy 34 (2009) 9379–9384
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Investigations on hydrogenation behaviour of CNT admixed Mg2Ni Sunil Kumar Pandey*, Rajesh Kumar Singh, O.N. Srivastava Hydrogen Energy Centre, Department of Physics, Banaras Hindu University, Varanasi, Uttar Pradesh 221005, India
article info
abstract
Article history:
The aim of the present paper is to report results on hydrogenation behaviour of the new
Received 19 July 2009
composite material Mg2Ni: CNT. Admixing of carbon nanotubes (CNT) in storage material
Received in revised form
Mg2Ni leads to noticeable enhancement in desorption kinetics as well as storage capacity.
10 September 2009
We have found that the composite material Mg2Ni–2 mole% CNT is the optimum material.
Accepted 24 September 2009
The Mg2Ni–CNT composite exhibits hydrogen desorption rate of 5.7 cc/g/min as against
Available online 22 October 2009
3.0 cc/g/min for Mg2Ni alone (enhancement of w 90%) and storage capacity of w 4.20 wt% in contrast to w3.20 wt% for Mg2Ni alone (increase of w 31%). Feasible mechanisms for the
Keywords:
enhancement of hydrogen desorption kinetics and storage capacity have been put forward. ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
Mg2Ni CNT Hydrogen storage Desorption kinetics X-ray diffraction TEM
1.
Introduction
Hydrogen is a clean fuel both for hot combustion as in IC Engines and cold combustion in Fuel Cells. The application of hydrogen has gained importance in recent times due to realization of the fact that global warming and resulting climate change due to CO2 build up is real. Coupled with this, the issues related with urban air pollution, depletion and price rise of fossil fuel (oil) and rather limited scope of biofuels has also led to upsurge of hydrogen energy related R&D efforts. It is now generally believed that hydrogen storage which cuts across the production, distribution, safety and application, forms the key factors for ‘‘hydrogen economy’’. Hydrogen storage material research focused on the synthesis and realization of high hydrogen storage capacity (>2 wt% and up to w6 wt %) is being pursued vigorously leading to their potential use in transportation applications.
Magnesium, which has hydrogen storage capacity of w7.6 wt% is a potential storage material. However, practical use of Mg as hydrogen storage material is not feasible due to sluggish kinetics and also high operation temperature [1–4]. Mg based alloys are better in this respect and form promising candidates for hydrogen storage. However, due to the low melting point (923 K) and high vapor pressure of magnesium, the preparation of Mg based alloys is difficult following the normal metallurgical processes [5]. For the preparation of Mg based hydrogen storage alloys, several methods such as conventional melting [6], mechanical alloying (MA) [7], combustion synthesis [8], melt spinning [9] etc have been developed. One of the most prominent Mg based intermetallic is Mg2Ni which shows high storage capacity of 3.20 wt%. The hydrogen desorption takes place at a temperature of w573 K. It is difficult to synthesize Mg2Ni intermetallic compound by the
* Corresponding author. Tel.: þ91 5422621060. E-mail address: [email protected] (S.K. Pandey). 0360-3199/$ – see front matter ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.09.077
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international journal of hydrogen energy 34 (2009) 9379–9384
conventional melting method because of the large difference in vapor pressures and melting points of Mg and Ni. It should be emphasized that the conventionally synthesized Mg2Ni cannot absorb hydrogen unless taken through more than 10 hydrogen absorption and desorption cycles at high temperatures (more than 573 K) [10]. The Mg2Ni intermetallic compound has been prepared by us through solid state sintering in inert gas (Ar) ambience. Carbon nanotubes (CNT) possess a unique morphology, electronic structure and catalytic activity and have high hydrogen storage capacity [11,12]. In recent studies, it has been shown that CNTs also have a positive and sustained effect on the sorption rate and capacity, as well as cycling capability of Mg – based materials [13]. We have also tried with other carbon forms as catalysts (i.e. graphite power, activated carbon etc.) in Mg2Ni. But we have not found significant increase in hydrogenation behaviors (Kinetics as well as storage capacity). The admixed carbon/CNT in such a small amount will not increase thermal contact of the sample with experimental setup to such extent which will effect so significantly on the hydrogenation/dehydrogenation behaviours. The impurities present in CNTs are very small in amount but it may have some influence on hydrogen storage properties. In the present investigation, it has been shown that admixing of Mg2Ni with x mole% CNT (x ¼ 0.0, 0.5, 1.0, 1.5, 2.0, 4.0 and 6.0) makes Mg2Ni a viable hydrogen storage material. The composite material Mg2Ni–2 mol% CNT has been found to correspond to the optimum material. This exhibits improved storage capacity of w4.20 wt%(H/M ¼ 1.57) as against w3.20 wt% for Mg2Ni alone. It also shows enhanced desorption kinetics w5.7 cc/g/min in contrast to w3 cc/g/min for Mg2Ni. Thus there is an increase of w 31% in storage capacity and an enhancement of w 90% in desorption kinetics. Feasible mechanism for the improvement of hydrogenation behavior with admixing of CNT has been described and discussed.
2.
Experimental
2.1.
Sample preparation
The samples corresponding to Mg2Ni have been fabricated through solid state reaction method at ambient pressure by using high purity of Mg (99.9%) and Ni (99.99%) powders. The powders were thoroughly mixed and pressed into pellet form (1 0.5 cm3). The pellet was then put in a steel reactor, fabricated in our laboratory. The reactor was evacuated to 106 torr. The aforesaid pellet was sintered in hydrogen atmosphere at temperature ranging between 723 and 823 K for various time periods (24 h at 723 K and then 48 h at 823 K). Even though Mg2Ni was formed as a result of above said sintering process, it invariably contained MgO as a secondary phase. The presence of MgO has been monitored through XRD. This is in keeping with the known results where the synthesis of Mg2Ni invariably shows the presence of MgO inclusions [14]. In order to avoid MgO, we carried out synthesis of Mg2Ni in argon atmosphere instead of vacuum. To achieve argon ambience, we introduced Ar (w1 atmosphere) after evacuation. Following this, the sintering was done at 723 and
823 K. It has been found that this synthesis route significantly lowered the formation of MgO. One gram of Mg2Ni was admixed with x mole% (x ¼ 0.0, 0.5, 1.0, 1.5, 2.0, 4.0, and 6.0) of carbon nanotube in argon atmosphere. Carbon nanotubes have been synthesized in our lab through spray pyrolysis technique. The details are given in our work [15,16]. Briefly speaking the synthesis process comprises of a nozzle attached to a ferrocene/benzene solution supply used for releasing the solution into quartz tube, mounted inside a temperature controlled cylindrical furnace. Ferrocene dissolved in benzene was injected into the quartz tube, using argon gas as a carrier gas. The temperature of the furnace was raised to 1173 K. The diameter of the CNT depends on the nozzle size and flow rate of the solution. The Fe catalyst from the as-synthesized CNTs was removed by placing the as grown CNTs overnight in concentrated nitric acid followed by washing of CNTs in deionised water. The CNTs were then dried in argon atmosphere by heating up to 573 K in an oven for 10 h. These CNTs, were found to be multi walled CNTs through transmission electron microscopy. The CNTs were admixed in Mg2Ni through grinding in an agate and pestle for two hours.
2.2. Structural characterization and hydrogenation behaviour X-ray diffraction (XRD) patterns were obtained on X’ Pert PRO PANalytical X-ray diffractometer using CuKa radiation ˚ , 40 kV, 30 mA). Transmission electron micro(l ¼ 1.5406 A scope (TEM) image were taken by means of Technai 20 G2 operated at 200 kV. Samples were dispersed and put on Cu grids for the TEM observation. The hydrogenation behavior was investigated by monitoring P-C isotherms and desorption kinetics. In each hydrogenation cycle hydrogen at pressure of w 80 atmosphere was used for charging Mg2Ni–x mole% CNT (x ¼ 0.0, 0.5, 1.0, 1.5, 2.0, 4.0, and 6.0) system. The reactor (inner dia: 2.186 cm) containing the material was evacuated up to 105 torr between each cycle of hydrogenation/dehydrogenation. The amount of hydrogen desorbed was monitored by volume displacement method using a Sievert’s type apparatus, developed in our laboratory [17] and also on a P-C-T measurement system procured from Advanced Material Corporation (AMC) Pittsburgh, USA. The temperatures employed correspond to 573, 623, 673 and 723 K. Representative desorption P-C-T characteristics of the composite materials at 573 K has been monitored. A similar technique was utilized to monitor the amount of hydrogen desorbed as a function of time to obtain the desorption kinetics.
3.
Results and discussion
3.1.
Structural and microstructural characterizations
We explored the structural characterization of the Mg2Ni, CNT admixed Mg2Ni in order to seek the correlation with the observed dehydrogenation and rehydrogenation characteristics. Fig. 1 (a) and (b) show X-ray diffractograms of the pure and 2 mole% CNT in Mg2Ni. Comparison of the XRD pattern
international journal of hydrogen energy 34 (2009) 9379–9384
Fig. 1 – X-ray diffractograms of the (a) as-synthesized (b) after dehydrogenation and (c) MgO free Mg2Ni–2 mole% CNT material (Inset shows that whereas in Figure (a) MgO peak is present, in Figure (c) which corresponds to sintering in argon ambience, MgO is nearly absent signifying formation of MgO free Mg2Ni).
with the known pattern of the Mg2Ni revealed that the material employed by us has the expected hexagonal crystal ˚ , c ¼ 13.274 A ˚ for Mg2Ni and structural phase with a ¼ 5.213 A ˚ ˚ a ¼ 5.237 A, c ¼ 13.369 A, for 2 mole% CNT admixed Mg2Ni. The above estimates of lattice parameters reveal that there is no noticeable change in lattice parameters of Mg2Ni, resulting from CNT admixing. This is expected, since CNT is not expected to get incorporated in Mg2Ni lattice either substitutionally or interstitially [18]. However, small change in lattice ˚ and Dc ¼ 0.095 A ˚ ) may be due to the parameters (Da ¼ 0.024 A Fe catalyst which is present in as grown CNT [19,20]. This Fe, may get incorporated at Ni sites in Mg2Ni while admixing CNT to Mg2Ni through mechanical mixing. Since Fe has large ˚ ) as compared to Ni (1.67 A ˚ ), substitution of atomic size (1.72 A Fe in place of Ni results in increase of lattice parameters. This appears to be the most feasible reason for small increase in lattice parameters. Synthesis in Ar ambience significantly reduces the formation of the impurity phase MgO. This can be seen from the insets of Fig. 1 (a) and Fig. 1(c). Thus sintering Mg and Ni in correct stoichiometry in Argon atmosphere can be taken to correspond to an optimum method for the synthesis of Mg2Ni with minimum MgO impurity phase. Representative TEM micrograph revealing the microstructural feature of 2 mole % CNT admixed Mg2Ni is also shown and discussed.
3.2.
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Fig. 2 – Dehydriding kinetics of Mg2Ni–x mole% CNT (x [ 0.0, 0.5, 1.0, 1.5, 2.0, 4.0, and 6.0) material at 573 K.
dehydrogenation kinetics of Mg2Ni–x mole% CNT. It can be noted by comparison of the P-C-T curves particularly the curves at 573 K (Figs. 2 and 3) for Mg2Ni and Mg2Ni–x mole% CNT that the dehydrogenation kinetics in composite material is higher as compared to the parent phase. The storage capacities were found to be 3.20, 3.26, 3.39, 3.48, 4.20, 3.77 and 3.34 wt% respectively. The P-C-T curves and desorption kinetics for Mg2Ni–x mole% CNT have been found to be reproducible. The storage capacity and kinetics for all the CNT admixed Mg2Ni materials studied in the present investigation are summarized in Table 1. As can be seen from Table 1, the storage capacity and desorption kinetics increases with increasing value of x. Admixing of CNT beyond x ¼ 2.0 leads to the decrease of the storage capacity even though desorption kinetics is still enhanced. Since storage capacity and dehydrogenation kinetics both are important, the material Mg2Ni–x mole% CNT
Hydrogenation behaviour
The known desorption temperature of Mg2Ni is 573 K [21].A representative example of the amount of desorbed hydrogen at 573 K for the Mg2Ni–x mole% CNT (x ¼ 0.0, 0.5, 1.0, 1.5, 2.0, 4.0, and 6.0) is shown in Fig. 2. For the sake of comparison, the experimental data from dehydriding cycle of Mg2Ni are also shown in this figure. These curves clearly illustrate the faster
Fig. 3 – Representative pressure-composition isotherms of the Mg2Ni–x mole% CNT (x [ 0.0, 0.5, 1.0, 1.5, 2.0, 4.0, and 6.0) material at 573 K.
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international journal of hydrogen energy 34 (2009) 9379–9384
Table 1 – Storage capacity and desorption kinetics of the Mg2Ni–mole% CNT hydrogen storage materials (x [ 0.0, 0.5, 1.0, 1.5, 2.0, 4.0 and 6.0) at 573 K. S. No.
1. 2. 3. 4. 5. 7. 8.
Composition
Temperature (K)
Storage Capacity (wt%)
Increament in Storage Capacity (%)
Desorption Time (min)
Desorption Kinetics (cc/g/min)
Increament in Kinetics %
Mg2Ni 0.5 mole% CNT–Mg2Ni 1 mole% CNT–Mg2Ni 1.5 mole% CNT–Mg2Ni 2 mole% CNT–Mg2Ni 4 mole% CNT–Mg2Ni 6 mole% CNT–Mg2Ni
573 573 573 573 573 573 573
3.20 3.26 3.39 3.48 4.20 3.77 3.34
Parent material 1.86 5.94 8.75 34.25 17.81 4.36
120 100 100 90 80 55 40
3 3.7 3.8 4.4 5.7 7.7 9.4
Parent material 23% 27% 47% 90% 157% 213%
with x ¼ 2 exhibiting storage capacity of w 4.20 wt% and desorption kinetics of w5.7 cc/g/min can be taken to correspond to the optimum material. As regards the decrease of hydrogen storage capacity, it is believed that with increasing CNT concentration the storage capacity will increase as long
as interstitial volume within Mg2Ni lattice remain intact even after addition of CNT. With increasing CNT concentration, if the interstitial storage volume gets compressed, the storage capacity will decrease. We next proceeded to monitor the effect of temperature on desorption kinetics and storage capacity. The P-C-T curves of Mg2Ni–2 mole% CNT at different temperatures i.e. at 573 K, 623 K, 673 K and 723 K are shown in Fig. 4(a). From this figure, it is clear that as temperature increases desorption plateau pressure get enhanced. The desorption kinetics of the optimum material at different temperatures are shown in Fig. 4(b) which show significant increment in kinetics with increasing temperature. The variation of desorption kinetics with temperature of optimum material i.e. Mg2Ni–2 mole% CNT is given in Table 2.
3.3.
Discussion
The results described above reveal that admixing CNT enhances (a) desorption kinetics w5.7 cc/g/min in contrast to w 3 cc/g/min (increase of w 90%) and (b) storage capacity w4.20 wt % as against 3.20 wt % for Mg2Ni (increase of w31%). As regards the enhancement in desorption kinetics, one primary reason may be presence of CNT. We explored details of microstructure of the Mg2Ni–CNT material. These explorations invariably suggest the presence of localized, surface features. A representative example is given in Fig. 5. This figure shows the TEM micrograph and the electron diffraction pattern of 2 mole% CNT-Mg2Ni after 10 hydrogenation/dehydrogenation cycles. The diffraction patterns shown in the inset revealed the presence of Mg2Ni and CNT in the microstructure. The localized regions produced by intersection of CNT with the surface can be seen in Fig. 5. These localized regions can be taken to be the sites where the admixed CNT intersects the surface of the composite. It can be seen that extent of localized regions range between w 10 nm to w 15 nm.
Table 2 – Effect of temperature on desorption kinetics of Mg2Ni–2 mole% CNT. S. No. Fig. 4 – (a) P-C-T of the Mg2Ni–2 mole% CNT admixed material at different temperatures. (b) Dehydriding kinetics of the Mg2Ni–2 mole% CNT admixed material at different temperatures.
1. 2. 3. 4.
Temperature
Desorption Kinetics
573 K 623 K 673 K 723 K
5.7 cc/g/min 11.4 cc/g/min 32.6 cc/g/min 57.1 cc/g/min
international journal of hydrogen energy 34 (2009) 9379–9384
Fig. 5 – TEM micrograph of 2 mole% CNT admixed Mg2Ni. Some of the CNTs are indicated by arrows (inset shows the corresponding SAED pattern showing the presence of CNT and Mg2Ni).
This is comparable with the observed diameter of the CNTs (MWNT) which were also found to be in this range. The CNTs which are very strong (they have higher tensile strength than steel), coming out from the interior of the material and intersecting the surface, will provide an easy passage route for direct transport through CNT tunnel of desorbed hydrogen, as is schematically shown in Fig. 6. This seems to be a potential reason for the faster kinetics of Mg2Ni–CNT composite material. It may be pointed out that even though CNTs which do not come up to the surface but are near the surface will be effective for the enhancement of desorption kinetics. Such ‘‘near the surface’’ CNTs will transport hydrogen directly close to the surface. The hydrogen will eventually reach the surface by diffusion through short spatial extent. Thus direct transport of desorbed hydrogen through CNT appears to be the most potential cause for the faster kinetics of the composite material Mg2Ni–CNT. In regard to increase in storage capacity (up to w 4.20 wt %), the enhancement from 3.20 wt% to 4.20 wt% occurred on CNT admixing. Therefore, increase in storage capacity apparently
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owes its origin to the CNTs present in the Mg2Ni–CNT composite. It may be pointed out that several workers [3,22– 26] have reported catalytic activity, adsorption and absorption of hydrogen in CNTs. However, these studies do not give any definite mechanism through which CNTs store hydrogen. It can thus be taken that the enhancement in capacity from 3.20 to 4.20 wt% (w31%) arises due to adsorption/absorption of H2/H in CNT. One possibility is that some H2 molecules get chemically adsorbed at the surface of the CNT. The other possibility is that hydrogen atoms formed at the surface of Mg2Ni through dissociation of H2 molecules get physically absorbed in CNT in addition to hydrogen atoms absorbed at interstitial sites in Mg2Ni. In both the cases i.e. the adsorbed/absorbed hydrogen will get released at the employed operation temperature w573 K. Based on the present experiments, it is not possible to exactly pin point how H2/H gets stored in CNTs in the composite. However, based on the known capability of CNTs in storing hydrogen, it can be taken that enhanced capacity of Mg2Ni–CNT up to 4.20 wt% arises here due to hydrogen stored in CNTs. Further studies on this are being carried out and results will be forthcoming.
4.
Conclusions
The hydrogen storage capacity and desorption kinetics of Mg2Ni can be markedly improved by admixing CNTs. A systematic investigation of Mg2Ni–CNT composite system has shown that the optimum hydrogen storage properties are achieved for Mg2Ni– 2 mole% CNT. Thus the composite material has the storage capacity of w4.20 wt% (for Mg2Ni alone it is w3.20 wt%, an increase of w 31%) and desorption kinetics 5.7 cc/g/min (for Mg2Ni alone it is 3 cc/g/min, enhancement of w 90%) at 573 K. The CNTs provide direct route for transport of desorbed hydrogen. The CNTs may also provide additional adsorption/absorption sites for hydrogen storage. Thus the presence of CNTs in Mg2Ni–CNT composite leads to the improvement in hydrogenation/dehydrogenation behaviors of the Mg2Ni–CNT composites.
Acknowledgements The authors will like to acknowledge Professor T. N. Veziroglu (President, IAHE Florida USA) and Professor M. Groll (Stuttgart, Germany) for helpful discussions. Financial assistance from Ministry of New and Renewable Energy (MNRE) and DST (UNANST), New Delhi (India) is gratefully acknowledged. One of the authors would like to thank the CSIR, New Delhi for the grant of Senior Research Fellowship.
references Fig. 6 – Schematic diagram of direct passage route for desorbing hydrogen from Mg2NiH4 (G represents grain of the material, black spheres represent hydrogen atoms, H atoms on surface will combine to form hydrogen molecules).
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[2] Jurczyk M, Smardz L, Nowak M, Jankowska E. Nanocrystalline LaNi5-type electrode materials for Ni-MHx batteries. J Solid State Chem 2003;171(1–2):30–7. [3] Bobet JL, Grigorova E, Khrussanova M, Khristov M, Stefanov P, Peshev P, et al. Hydrogen sorption properties of graphite-modified magnesium nanocomposites prepared by ball-milling. J Alloys Comp 2004;366:298–302. [4] Eu CZ, Wang P, Yao X, Liu C, Chen DM, Lu GQ, et al. Effect of carbon/noncarbon addition on hydrogen storage behaviors of magnesium hydride. J Alloys Comp 2006;414:259–64. [5] Precherom A, Wlter J-M, Shlaphoch L. Hydrogen in inermetallic compounds, vol. II. Berlin: Sringer-Verlog; 1992. 20. [6] Hoffman KC, Reilly JJ, Salzano FJ, Waide CH, Wiswall RH, Winsche WE. Metal hydride storage for mobile and stationary applications. Int J Hydrogen Energy 1976;2(1): 133–51. [7] Liang G, Boily S, Hout J, Neste A, Schulz R. Hydrogen absorption properties of a mechanically milled Mg-50 wt.% LaNi5 composite. J Alloys Compd 1998;268:302–7. [8] Li Liquan, Akiyama T, Kabutomori T, Terao K, Yagi J. In situ X-ray diffraction study of the hydriding combustion synthesis of Mg2NiH4. J Alloys Compd 1998;281(2):175–80. [9] Spassov T, Koser U. Hydrogenation of amorphous and nanocrystalline Mg-based alloys. J Alloys Compd 1999;287(1–2):243–50. [10] Huaiyu S, Hairou X, Yuntau W, Xingguo L. Preparation and hydrogen storage properties of Mg2Ni intermetallic nanoparticles. Nanotechnology 2004;15(3):269–74. [11] Nechaev YuS, Alexeeva OK, Oechsner A. On the hydrogen multilayer intercalation in carbonaceous nanostructures: relevance for development of super-adsorbents for fuel-cellpowered vehicles. J Nanosci Nanotechnol 2009;9(6):3949–58. [12] Nechaev YuS. The nature, kinetics and ultimate storage capacity of hydrogen sorption by carbon nanostructures. Physics-Uspekhi 2006;49(6):563–91. [13] Guo ZP, Huang GZ, Konstantinov K, Liu HK, Dou SX. Electrochemical hydrogen storage properties of nonstoichiometric amorphous MgNi1þx–carbon composites (x ¼ 0.05–0.3). Int J Hydrogen Energy 2006;31(14):2032–9. [14] Liu Z, Lie Z. Cyclic hydrogen storage properties of Mg milled with nickel nano-powders and MnO2. J Alloys Comp 2007; 443(1–2):121–4.
[15] Yadav RM, Srivastava A, Srivastava ON. Synthesis of bamboo-shaped carbon-nitrogen nanotubes using acetonitrile-ferrocene precursor. J Nanosci Nanotechnol 2004;4(1–2):719–21. [16] Mishra PR, Awasthi K, Srivastava ON. Formation and microstructural characterization of coaxial carbon cylinders consisting of aligned carbon nanotube stacks. J Nanosci Nanotech 2007;7(1):1815–9. [17] Pukazhselvan D, Gupta BK, Srivastava A, Srivastava ON. Investigations on hydrogen storage behavior of CNT doped NaAlH4. J Alloys Compd 2005;403(1–2):312–7. [18] Pukazhselvan D, Hudson MSL, Gupta BK, Shaz MA, Srivastava ON. Investigations on the desorption kinetics of Mm-doped NaAlH4. J Alloys Compd 2007;439(1–2). 243–248. [19] Huot J, Liang G, Boily S, Naste AV, Schulz R. Structural study and hydrogen sorption kinetics of ball-milled magnesium hydride. J Alloys Compd 1999;293–295:495–500. [20] Wu CZ, Wang P, Yao XD, Liu C, Chen DM, Lu GQ, et al. Effects of SWNT and metallic catalyst on hydrogen absorption/ desorption performance of MgH2. J Phys Chem B 2005; 109(47):22217–21. [21] Hima Kumar L, Viswanathan B, Srinivasa Murthy S. Hydrogen absorption by Mg2Ni prepared by polyol reduction. J Alloys Compd 2007;461(1–2):72–6. [22] Tsuchiya S, Yamaguchi H, Nakashima Y, Sakata Y, Imamura H. Catalytic isomerization of n-butenes and hydrogenation over a samarium oxide and Ga2O3- modified samarium oxides. J Alloys Compd 1999;193(1–2):52–5. [23] Grigorova E, Khristov M, Khrussanova M, Bobet JL, Peshev P. Effect of additives on the hydrogen sorption properties of mechanically alloyed composites based on Mg and Mg2Ni. Int J Hydrogen Energy 2005;30(10):1099–7705. [24] Wu CZ, Wang P, Yao X, Liu C, Chen DM, Lu GQ, et al. Hydrogen storage properties of MgH2/SWNT composite prepared by ball milling. J Alloys Compd 2006;420(1–2): 278–82. [25] Luo Y, Wang P, Ma LP, Cheng HM. Enhanced hydrogen storage properties of MgH2 co-catalyzed with NbF5 and single-walled carbon nanotubes. Scr Mater 2007;56(9): 765–8. [26] Xie DH, Li P, Zeng CX, Sun JW, Qu XH. Effect of substitution of Nd for Mg on the hydrogen storage properties of Mg2Ni alloy. J Alloys and Compd 2009;478(1–2):94–6.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Investigations on hydrogenation behaviour of CNT admixed Mg2Ni Sunil Kumar Pandey*, Rajesh Kumar Singh, O.N. Srivastava Hydrogen Energy Centre, Department of Physics, Banaras Hindu University, Varanasi, Uttar Pradesh 221005, India
article info
abstract
Article history:
The aim of the present paper is to report results on hydrogenation behaviour of the new
Received 19 July 2009
composite material Mg2Ni: CNT. Admixing of carbon nanotubes (CNT) in storage material
Received in revised form
Mg2Ni leads to noticeable enhancement in desorption kinetics as well as storage capacity.
10 September 2009
We have found that the composite material Mg2Ni–2 mole% CNT is the optimum material.
Accepted 24 September 2009
The Mg2Ni–CNT composite exhibits hydrogen desorption rate of 5.7 cc/g/min as against
Available online 22 October 2009
3.0 cc/g/min for Mg2Ni alone (enhancement of w 90%) and storage capacity of w 4.20 wt% in contrast to w3.20 wt% for Mg2Ni alone (increase of w 31%). Feasible mechanisms for the
Keywords:
enhancement of hydrogen desorption kinetics and storage capacity have been put forward. ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
Mg2Ni CNT Hydrogen storage Desorption kinetics X-ray diffraction TEM
1.
Introduction
Hydrogen is a clean fuel both for hot combustion as in IC Engines and cold combustion in Fuel Cells. The application of hydrogen has gained importance in recent times due to realization of the fact that global warming and resulting climate change due to CO2 build up is real. Coupled with this, the issues related with urban air pollution, depletion and price rise of fossil fuel (oil) and rather limited scope of biofuels has also led to upsurge of hydrogen energy related R&D efforts. It is now generally believed that hydrogen storage which cuts across the production, distribution, safety and application, forms the key factors for ‘‘hydrogen economy’’. Hydrogen storage material research focused on the synthesis and realization of high hydrogen storage capacity (>2 wt% and up to w6 wt %) is being pursued vigorously leading to their potential use in transportation applications.
Magnesium, which has hydrogen storage capacity of w7.6 wt% is a potential storage material. However, practical use of Mg as hydrogen storage material is not feasible due to sluggish kinetics and also high operation temperature [1–4]. Mg based alloys are better in this respect and form promising candidates for hydrogen storage. However, due to the low melting point (923 K) and high vapor pressure of magnesium, the preparation of Mg based alloys is difficult following the normal metallurgical processes [5]. For the preparation of Mg based hydrogen storage alloys, several methods such as conventional melting [6], mechanical alloying (MA) [7], combustion synthesis [8], melt spinning [9] etc have been developed. One of the most prominent Mg based intermetallic is Mg2Ni which shows high storage capacity of 3.20 wt%. The hydrogen desorption takes place at a temperature of w573 K. It is difficult to synthesize Mg2Ni intermetallic compound by the
* Corresponding author. Tel.: þ91 5422621060. E-mail address: [email protected] (S.K. Pandey). 0360-3199/$ – see front matter ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.09.077
9380
international journal of hydrogen energy 34 (2009) 9379–9384
conventional melting method because of the large difference in vapor pressures and melting points of Mg and Ni. It should be emphasized that the conventionally synthesized Mg2Ni cannot absorb hydrogen unless taken through more than 10 hydrogen absorption and desorption cycles at high temperatures (more than 573 K) [10]. The Mg2Ni intermetallic compound has been prepared by us through solid state sintering in inert gas (Ar) ambience. Carbon nanotubes (CNT) possess a unique morphology, electronic structure and catalytic activity and have high hydrogen storage capacity [11,12]. In recent studies, it has been shown that CNTs also have a positive and sustained effect on the sorption rate and capacity, as well as cycling capability of Mg – based materials [13]. We have also tried with other carbon forms as catalysts (i.e. graphite power, activated carbon etc.) in Mg2Ni. But we have not found significant increase in hydrogenation behaviors (Kinetics as well as storage capacity). The admixed carbon/CNT in such a small amount will not increase thermal contact of the sample with experimental setup to such extent which will effect so significantly on the hydrogenation/dehydrogenation behaviours. The impurities present in CNTs are very small in amount but it may have some influence on hydrogen storage properties. In the present investigation, it has been shown that admixing of Mg2Ni with x mole% CNT (x ¼ 0.0, 0.5, 1.0, 1.5, 2.0, 4.0 and 6.0) makes Mg2Ni a viable hydrogen storage material. The composite material Mg2Ni–2 mol% CNT has been found to correspond to the optimum material. This exhibits improved storage capacity of w4.20 wt%(H/M ¼ 1.57) as against w3.20 wt% for Mg2Ni alone. It also shows enhanced desorption kinetics w5.7 cc/g/min in contrast to w3 cc/g/min for Mg2Ni. Thus there is an increase of w 31% in storage capacity and an enhancement of w 90% in desorption kinetics. Feasible mechanism for the improvement of hydrogenation behavior with admixing of CNT has been described and discussed.
2.
Experimental
2.1.
Sample preparation
The samples corresponding to Mg2Ni have been fabricated through solid state reaction method at ambient pressure by using high purity of Mg (99.9%) and Ni (99.99%) powders. The powders were thoroughly mixed and pressed into pellet form (1 0.5 cm3). The pellet was then put in a steel reactor, fabricated in our laboratory. The reactor was evacuated to 106 torr. The aforesaid pellet was sintered in hydrogen atmosphere at temperature ranging between 723 and 823 K for various time periods (24 h at 723 K and then 48 h at 823 K). Even though Mg2Ni was formed as a result of above said sintering process, it invariably contained MgO as a secondary phase. The presence of MgO has been monitored through XRD. This is in keeping with the known results where the synthesis of Mg2Ni invariably shows the presence of MgO inclusions [14]. In order to avoid MgO, we carried out synthesis of Mg2Ni in argon atmosphere instead of vacuum. To achieve argon ambience, we introduced Ar (w1 atmosphere) after evacuation. Following this, the sintering was done at 723 and
823 K. It has been found that this synthesis route significantly lowered the formation of MgO. One gram of Mg2Ni was admixed with x mole% (x ¼ 0.0, 0.5, 1.0, 1.5, 2.0, 4.0, and 6.0) of carbon nanotube in argon atmosphere. Carbon nanotubes have been synthesized in our lab through spray pyrolysis technique. The details are given in our work [15,16]. Briefly speaking the synthesis process comprises of a nozzle attached to a ferrocene/benzene solution supply used for releasing the solution into quartz tube, mounted inside a temperature controlled cylindrical furnace. Ferrocene dissolved in benzene was injected into the quartz tube, using argon gas as a carrier gas. The temperature of the furnace was raised to 1173 K. The diameter of the CNT depends on the nozzle size and flow rate of the solution. The Fe catalyst from the as-synthesized CNTs was removed by placing the as grown CNTs overnight in concentrated nitric acid followed by washing of CNTs in deionised water. The CNTs were then dried in argon atmosphere by heating up to 573 K in an oven for 10 h. These CNTs, were found to be multi walled CNTs through transmission electron microscopy. The CNTs were admixed in Mg2Ni through grinding in an agate and pestle for two hours.
2.2. Structural characterization and hydrogenation behaviour X-ray diffraction (XRD) patterns were obtained on X’ Pert PRO PANalytical X-ray diffractometer using CuKa radiation ˚ , 40 kV, 30 mA). Transmission electron micro(l ¼ 1.5406 A scope (TEM) image were taken by means of Technai 20 G2 operated at 200 kV. Samples were dispersed and put on Cu grids for the TEM observation. The hydrogenation behavior was investigated by monitoring P-C isotherms and desorption kinetics. In each hydrogenation cycle hydrogen at pressure of w 80 atmosphere was used for charging Mg2Ni–x mole% CNT (x ¼ 0.0, 0.5, 1.0, 1.5, 2.0, 4.0, and 6.0) system. The reactor (inner dia: 2.186 cm) containing the material was evacuated up to 105 torr between each cycle of hydrogenation/dehydrogenation. The amount of hydrogen desorbed was monitored by volume displacement method using a Sievert’s type apparatus, developed in our laboratory [17] and also on a P-C-T measurement system procured from Advanced Material Corporation (AMC) Pittsburgh, USA. The temperatures employed correspond to 573, 623, 673 and 723 K. Representative desorption P-C-T characteristics of the composite materials at 573 K has been monitored. A similar technique was utilized to monitor the amount of hydrogen desorbed as a function of time to obtain the desorption kinetics.
3.
Results and discussion
3.1.
Structural and microstructural characterizations
We explored the structural characterization of the Mg2Ni, CNT admixed Mg2Ni in order to seek the correlation with the observed dehydrogenation and rehydrogenation characteristics. Fig. 1 (a) and (b) show X-ray diffractograms of the pure and 2 mole% CNT in Mg2Ni. Comparison of the XRD pattern
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Fig. 1 – X-ray diffractograms of the (a) as-synthesized (b) after dehydrogenation and (c) MgO free Mg2Ni–2 mole% CNT material (Inset shows that whereas in Figure (a) MgO peak is present, in Figure (c) which corresponds to sintering in argon ambience, MgO is nearly absent signifying formation of MgO free Mg2Ni).
with the known pattern of the Mg2Ni revealed that the material employed by us has the expected hexagonal crystal ˚ , c ¼ 13.274 A ˚ for Mg2Ni and structural phase with a ¼ 5.213 A ˚ ˚ a ¼ 5.237 A, c ¼ 13.369 A, for 2 mole% CNT admixed Mg2Ni. The above estimates of lattice parameters reveal that there is no noticeable change in lattice parameters of Mg2Ni, resulting from CNT admixing. This is expected, since CNT is not expected to get incorporated in Mg2Ni lattice either substitutionally or interstitially [18]. However, small change in lattice ˚ and Dc ¼ 0.095 A ˚ ) may be due to the parameters (Da ¼ 0.024 A Fe catalyst which is present in as grown CNT [19,20]. This Fe, may get incorporated at Ni sites in Mg2Ni while admixing CNT to Mg2Ni through mechanical mixing. Since Fe has large ˚ ) as compared to Ni (1.67 A ˚ ), substitution of atomic size (1.72 A Fe in place of Ni results in increase of lattice parameters. This appears to be the most feasible reason for small increase in lattice parameters. Synthesis in Ar ambience significantly reduces the formation of the impurity phase MgO. This can be seen from the insets of Fig. 1 (a) and Fig. 1(c). Thus sintering Mg and Ni in correct stoichiometry in Argon atmosphere can be taken to correspond to an optimum method for the synthesis of Mg2Ni with minimum MgO impurity phase. Representative TEM micrograph revealing the microstructural feature of 2 mole % CNT admixed Mg2Ni is also shown and discussed.
3.2.
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Fig. 2 – Dehydriding kinetics of Mg2Ni–x mole% CNT (x [ 0.0, 0.5, 1.0, 1.5, 2.0, 4.0, and 6.0) material at 573 K.
dehydrogenation kinetics of Mg2Ni–x mole% CNT. It can be noted by comparison of the P-C-T curves particularly the curves at 573 K (Figs. 2 and 3) for Mg2Ni and Mg2Ni–x mole% CNT that the dehydrogenation kinetics in composite material is higher as compared to the parent phase. The storage capacities were found to be 3.20, 3.26, 3.39, 3.48, 4.20, 3.77 and 3.34 wt% respectively. The P-C-T curves and desorption kinetics for Mg2Ni–x mole% CNT have been found to be reproducible. The storage capacity and kinetics for all the CNT admixed Mg2Ni materials studied in the present investigation are summarized in Table 1. As can be seen from Table 1, the storage capacity and desorption kinetics increases with increasing value of x. Admixing of CNT beyond x ¼ 2.0 leads to the decrease of the storage capacity even though desorption kinetics is still enhanced. Since storage capacity and dehydrogenation kinetics both are important, the material Mg2Ni–x mole% CNT
Hydrogenation behaviour
The known desorption temperature of Mg2Ni is 573 K [21].A representative example of the amount of desorbed hydrogen at 573 K for the Mg2Ni–x mole% CNT (x ¼ 0.0, 0.5, 1.0, 1.5, 2.0, 4.0, and 6.0) is shown in Fig. 2. For the sake of comparison, the experimental data from dehydriding cycle of Mg2Ni are also shown in this figure. These curves clearly illustrate the faster
Fig. 3 – Representative pressure-composition isotherms of the Mg2Ni–x mole% CNT (x [ 0.0, 0.5, 1.0, 1.5, 2.0, 4.0, and 6.0) material at 573 K.
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Table 1 – Storage capacity and desorption kinetics of the Mg2Ni–mole% CNT hydrogen storage materials (x [ 0.0, 0.5, 1.0, 1.5, 2.0, 4.0 and 6.0) at 573 K. S. No.
1. 2. 3. 4. 5. 7. 8.
Composition
Temperature (K)
Storage Capacity (wt%)
Increament in Storage Capacity (%)
Desorption Time (min)
Desorption Kinetics (cc/g/min)
Increament in Kinetics %
Mg2Ni 0.5 mole% CNT–Mg2Ni 1 mole% CNT–Mg2Ni 1.5 mole% CNT–Mg2Ni 2 mole% CNT–Mg2Ni 4 mole% CNT–Mg2Ni 6 mole% CNT–Mg2Ni
573 573 573 573 573 573 573
3.20 3.26 3.39 3.48 4.20 3.77 3.34
Parent material 1.86 5.94 8.75 34.25 17.81 4.36
120 100 100 90 80 55 40
3 3.7 3.8 4.4 5.7 7.7 9.4
Parent material 23% 27% 47% 90% 157% 213%
with x ¼ 2 exhibiting storage capacity of w 4.20 wt% and desorption kinetics of w5.7 cc/g/min can be taken to correspond to the optimum material. As regards the decrease of hydrogen storage capacity, it is believed that with increasing CNT concentration the storage capacity will increase as long
as interstitial volume within Mg2Ni lattice remain intact even after addition of CNT. With increasing CNT concentration, if the interstitial storage volume gets compressed, the storage capacity will decrease. We next proceeded to monitor the effect of temperature on desorption kinetics and storage capacity. The P-C-T curves of Mg2Ni–2 mole% CNT at different temperatures i.e. at 573 K, 623 K, 673 K and 723 K are shown in Fig. 4(a). From this figure, it is clear that as temperature increases desorption plateau pressure get enhanced. The desorption kinetics of the optimum material at different temperatures are shown in Fig. 4(b) which show significant increment in kinetics with increasing temperature. The variation of desorption kinetics with temperature of optimum material i.e. Mg2Ni–2 mole% CNT is given in Table 2.
3.3.
Discussion
The results described above reveal that admixing CNT enhances (a) desorption kinetics w5.7 cc/g/min in contrast to w 3 cc/g/min (increase of w 90%) and (b) storage capacity w4.20 wt % as against 3.20 wt % for Mg2Ni (increase of w31%). As regards the enhancement in desorption kinetics, one primary reason may be presence of CNT. We explored details of microstructure of the Mg2Ni–CNT material. These explorations invariably suggest the presence of localized, surface features. A representative example is given in Fig. 5. This figure shows the TEM micrograph and the electron diffraction pattern of 2 mole% CNT-Mg2Ni after 10 hydrogenation/dehydrogenation cycles. The diffraction patterns shown in the inset revealed the presence of Mg2Ni and CNT in the microstructure. The localized regions produced by intersection of CNT with the surface can be seen in Fig. 5. These localized regions can be taken to be the sites where the admixed CNT intersects the surface of the composite. It can be seen that extent of localized regions range between w 10 nm to w 15 nm.
Table 2 – Effect of temperature on desorption kinetics of Mg2Ni–2 mole% CNT. S. No. Fig. 4 – (a) P-C-T of the Mg2Ni–2 mole% CNT admixed material at different temperatures. (b) Dehydriding kinetics of the Mg2Ni–2 mole% CNT admixed material at different temperatures.
1. 2. 3. 4.
Temperature
Desorption Kinetics
573 K 623 K 673 K 723 K
5.7 cc/g/min 11.4 cc/g/min 32.6 cc/g/min 57.1 cc/g/min
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Fig. 5 – TEM micrograph of 2 mole% CNT admixed Mg2Ni. Some of the CNTs are indicated by arrows (inset shows the corresponding SAED pattern showing the presence of CNT and Mg2Ni).
This is comparable with the observed diameter of the CNTs (MWNT) which were also found to be in this range. The CNTs which are very strong (they have higher tensile strength than steel), coming out from the interior of the material and intersecting the surface, will provide an easy passage route for direct transport through CNT tunnel of desorbed hydrogen, as is schematically shown in Fig. 6. This seems to be a potential reason for the faster kinetics of Mg2Ni–CNT composite material. It may be pointed out that even though CNTs which do not come up to the surface but are near the surface will be effective for the enhancement of desorption kinetics. Such ‘‘near the surface’’ CNTs will transport hydrogen directly close to the surface. The hydrogen will eventually reach the surface by diffusion through short spatial extent. Thus direct transport of desorbed hydrogen through CNT appears to be the most potential cause for the faster kinetics of the composite material Mg2Ni–CNT. In regard to increase in storage capacity (up to w 4.20 wt %), the enhancement from 3.20 wt% to 4.20 wt% occurred on CNT admixing. Therefore, increase in storage capacity apparently
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owes its origin to the CNTs present in the Mg2Ni–CNT composite. It may be pointed out that several workers [3,22– 26] have reported catalytic activity, adsorption and absorption of hydrogen in CNTs. However, these studies do not give any definite mechanism through which CNTs store hydrogen. It can thus be taken that the enhancement in capacity from 3.20 to 4.20 wt% (w31%) arises due to adsorption/absorption of H2/H in CNT. One possibility is that some H2 molecules get chemically adsorbed at the surface of the CNT. The other possibility is that hydrogen atoms formed at the surface of Mg2Ni through dissociation of H2 molecules get physically absorbed in CNT in addition to hydrogen atoms absorbed at interstitial sites in Mg2Ni. In both the cases i.e. the adsorbed/absorbed hydrogen will get released at the employed operation temperature w573 K. Based on the present experiments, it is not possible to exactly pin point how H2/H gets stored in CNTs in the composite. However, based on the known capability of CNTs in storing hydrogen, it can be taken that enhanced capacity of Mg2Ni–CNT up to 4.20 wt% arises here due to hydrogen stored in CNTs. Further studies on this are being carried out and results will be forthcoming.
4.
Conclusions
The hydrogen storage capacity and desorption kinetics of Mg2Ni can be markedly improved by admixing CNTs. A systematic investigation of Mg2Ni–CNT composite system has shown that the optimum hydrogen storage properties are achieved for Mg2Ni– 2 mole% CNT. Thus the composite material has the storage capacity of w4.20 wt% (for Mg2Ni alone it is w3.20 wt%, an increase of w 31%) and desorption kinetics 5.7 cc/g/min (for Mg2Ni alone it is 3 cc/g/min, enhancement of w 90%) at 573 K. The CNTs provide direct route for transport of desorbed hydrogen. The CNTs may also provide additional adsorption/absorption sites for hydrogen storage. Thus the presence of CNTs in Mg2Ni–CNT composite leads to the improvement in hydrogenation/dehydrogenation behaviors of the Mg2Ni–CNT composites.
Acknowledgements The authors will like to acknowledge Professor T. N. Veziroglu (President, IAHE Florida USA) and Professor M. Groll (Stuttgart, Germany) for helpful discussions. Financial assistance from Ministry of New and Renewable Energy (MNRE) and DST (UNANST), New Delhi (India) is gratefully acknowledged. One of the authors would like to thank the CSIR, New Delhi for the grant of Senior Research Fellowship.
references Fig. 6 – Schematic diagram of direct passage route for desorbing hydrogen from Mg2NiH4 (G represents grain of the material, black spheres represent hydrogen atoms, H atoms on surface will combine to form hydrogen molecules).
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